1. Field of the Invention
The present invention relates to image-forming apparatuses, such as color printers and color copy machines.
2. Description of the Related Art
Electrophotographic color image-forming apparatuses having various structures are known. For example, a so-called single path structure includes four photosensitive drums, four optical units, yellow (hereinafter Y), magenta (hereinafter M), cyan (hereinafter C), and black (hereinafter K) developing devices, and a single conveyor belt. In this structure, each of the Y, M, C, and K developing devices has a dedicated photosensitive drum and an optical unit, and a sheet of paper held by the conveyor belt successively passes the Y, M, C, and K photosensitive drums, where Y, M, C, and K images are transferred onto the sheet of paper. In this structure, processes of forming the Y, M, C, and K images are performed in parallel, and therefore the process speed can be increased. However, since four photosensitive drums and four optical units like laser scanners are necessary, it is difficult to reduce the size of the apparatus. In addition, since the images of different colors are formed at different positions until they are transferred onto the sheet, it is difficult to reduce color shifts.
On the other hand, a so-called four path structure includes a single photosensitive drum, a single optical unit, Y, M, C, and K developing devices, and a single intermediate transfer member. In this structure, the Y, M, C, and K developing devices successively come into contact with the photosensitive drum and transfer toner images of respective colors onto the intermediate transfer member. Accordingly, images of four colors are superimposed on the intermediate transfer member, and are then simultaneously transferred onto a sheet of paper. Since only one photosensitive drum and one optical unit like a laser scanner are necessary, the size of the apparatus can be reduced. In addition, since the images of four colors are formed at the same position on the photosensitive drum and the intermediate transfer member, color shifts do not easily occur. However, since a similar process is repeated four times, it is difficult to increase the print speed.
Accordingly, a two-path structure including two photosensitive drums, two optical units, Y, M, C, and K developing devices, and a single intermediate transfer member is known as an intermediate structure of the above-described structures. In this structure, for example, developing devices for Y (first color) and C (second color) are disposed around one of the photosensitive drums, and developing devices for M (third color) and K (fourth color) are disposed around the other one of the photosensitive drums. During a first turn of the intermediate transfer member, the Y and M developing devices are brought into contact with the respective photosensitive drums to transfer Y and M toner images onto the intermediate transfer member. During a second turn of the intermediate transfer member, the C and K developing devices are brought into contact with the respective photosensitive drums to transfer C and K toner images onto the intermediate transfer member such that the C and K toner images are superimposed on the images of Y and M. Then, all of the toner images are simultaneously transferred onto a sheet of paper. This structure has intermediate characteristics between those of the single-path and four-path structures.
Various methods are suggested for setting timing to start writing images of different colors in electrophotographic printers having the two-path structure. Here, it is assumed that Y and M images are formed by upstream and downstream image-forming units, respectively, during the first turn of the intermediate transfer member and C and K images are formed by the upstream and downstream image-forming units, respectively, during the second turn of the intermediate transfer member. Accordingly, the Y, M, C, and K images are formed in that order.
FIG. 9 is a diagram showing a known two-path structure. The two-path structure includes upstream and downstream photosensitive drums 1a and 1b and an intermediate transfer belt 4. An optical sensor 50 including a light emitter section and a light receiver section is provided for detecting horizontal lines on the intermediate transfer belt 4. The optical sensor 50 is disposed between an upstream image-forming unit and a downstream image-forming unit. FIG. 10 is a diagram showing a detection pattern on the intermediate transfer belt 4 of the known structure. Before an image-forming process is started, the photosensitive drums 1a and 1b are simultaneously subjected to exposure to form a detection pattern including a Y horizontal line 51 and an M horizontal line 52 on the intermediate transfer belt 4. FIG. 11 is a diagram showing detection timing of the detection pattern on the intermediate transfer belt 4 of the known structure. When the intermediate transfer belt 4 moves, first, the optical sensor 50 detects the Y horizontal line 51 (denoted by 53 in FIG. 11). Then, by the time the intermediate transfer belt 4 rotates by substantially one turn, the M horizontal line is detected (denoted by 54 in FIG. 11) and the Y horizontal line is detected for the second time (denoted by 55 in FIG. 11). Then, the detection pattern on the intermediate transfer belt 4 is cleaned. Since the exposure of the Y horizontal line 51 and the exposure of the M horizontal line 52 are performed at the same time, a gap between the Y and M horizontal lines 51 and 52 corresponds to a distance between the upstream and downstream image-forming units. The time to start writing in the downstream image-forming unit with respect to that in the upstream image-forming unit, that is, the time to form the M image with respect to the Y image or the time to form the K image with respect to the C image is determined on the basis of an interval (denoted by ta in FIG. 11) between the time at which the M horizontal line is detected (denoted by 54 in FIG. 11) and the time at which the Y horizontal line is detected the second time (denoted by 55 in FIG. 11). An interval between the time at which the Y horizontal line is detected the first time (denoted by 53 in FIG. 11) and the time at which Y horizontal line is detected the second time (denoted by 55 in FIG. 11) corresponds to the peripheral length of the intermediate transfer belt 4. Accordingly, the time to start writing in the upstream image-forming unit in the second turn with respect to that in the first turn, that is, the time to form the C image with respect to the Y image is determined on the basis of the interval (denoted by tb in FIG. 11) between the time at which the Y horizontal line is detected the first time (denoted by 53 in FIG. 11) and the time at which the Y horizontal line is detected the second time (denoted by 55 in FIG. 11). The time to start forming the Y image is determined on the basis of the time at which a sheet of paper is conveyed and the positional relationship between the toner image on the intermediate transfer belt 4 and a transfer area of the sheet of paper in which the toner image is transferred.
However, the above-described known structure has the following problems. That is, the color shifts cause a problem (becomes noticeable or apparent) even when they are very small relative to the gap between the upstream and downstream image-forming units and the peripheral length of the intermediate transfer belt. In general, the gap between the upstream and downstream image-forming units and the peripheral length of the intermediate transfer belt are about several tens to several hundreds of millimeters, while even a color shift about 150 μm or less causes a problem. Therefore, it is difficult to detect errors in time intervals corresponding to about 150 μm or less with high accuracy from detection results of time intervals corresponding to the movement of the intermediate transfer belt of about several tens to several hundreds of millimeters. In addition, the behavior of the intermediate transfer belt in the detecting section and the behavior of the intermediate transfer belt in the image-forming units are not always the same. Therefore, the time interval corresponding to the gap between the upstream and downstream image-forming units detected by the detecting unit from the movement of the intermediate transfer belt is different from the actual time which elapses while the intermediate transfer belt moves between the upstream and downstream image-forming units. Similarly, the time interval corresponding to the peripheral length of the intermediate transfer belt detected by the detecting unit from the movement of the intermediate transfer belt is different from the actual time interval between the times at which the intermediate transfer belt passes through the upstream image-forming unit in the first and second turns. Therefore, it is difficult to detect the color shifts between different colors with high accuracy by the known method.
FIG. 12 is a diagram showing an example of a color shift. The arrow shows a conveying direction of the intermediate transfer belt, and reference numerals 56 and 57 denote horizontal lines of different colors. These horizontal lines are normally formed at the same position in the conveying direction of the intermediate transfer belt. The color shift shown in FIG. 12 occurs when image-forming timing in the downstream image-forming unit with respect to that in the upstream image-forming unit is not accurately controlled or when image-forming timing in the second turn of the intermediate transfer belt with respect to that in the first turn is not accurately controlled.